drm/i915: Fix i915_sg_page_sizes to record dma segments rather than physical pages

All users of this function actually want the dma segment sizes, but that's
not what's calculated. Fix that and rename the function to
i915_sg_dma_sizes to reflect what's calculated.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210601074654.3103-4-thomas.hellstrom@linux.intel.com